1. Field of the Invention
This invention relates to a method of installing an optical fibre unit in a tube. The term xe2x80x9coptical fibre unitxe2x80x9d is used herein to describe an article which comprises a single optical fibre, or a plurality of optical fibres assembled to form a multi-fibre unit.
2. Description of Related Art
At the present time there are broadly two approaches to the in situ installation of optical fibre units in tubes. The first, and older, approach is to pull the fibre unit into the tube using an elongate pulling member. Fibre units which are to be installed in this way are heavyweight units/cables, and are provided with a substantial measure of reinforcement, for example by surrounding the fibre or fibre bundles with a rugged sheath, and/or using a steel, Kevlar, or similar strength member. (xe2x80x9cKevlarxe2x80x9d is a Registered Trade Mark for a polyarylimide material). The philosophy behind this approach is that pulling exerts substantial forces on the optical fibre unit, and that such reinforcement is needed to withstand those forces. It must be borne in mind that the installation force within a fibre unit being installed by pulling increases progressively from the trailing end of the unit to the leading end. The theory has been that for the installation force at the trailing end to be sufficient, the force at the leading end must be so high that, in the absence of such reinforcement, the fibre will be damaged by it mechanically, or even if it survives mechanically, its optical properties will be adversely affected.
The second approach is to use a lightweight fibre unit and install it using a flow of fluid in a tube, preferably a flow of a gas, and more preferably, a flow of air. Such a method is described, for example, in EP-A-108590. Using a fluid has the effect that the propelling force exerted on the fibre unit is, at least in theory, substantially the same throughout its length, and this force is much less than the maximum force present in a fibre unit which is being installed by pulling. Accordingly, significant reinforcement of the fibre is not required, and indeed would be undesirable since it would increase the weight per unit length of the fibre and hence the fluid drag required to achieve installation.
However, although the blowing approach has attractions, and is being successfully used, it has a number of disadvantages. Thus, the distance over which, and speed with which, fibres can be installed is limited for any given tube diameter by the pressure drop and flow restrictions imposed thereby. Increasing the tube diameter to reduce these effects means that the tubes or bundles thereof occupy more space in the network and are thus more difficult to manage, particularly when a number of such bundles are present, for example at exchange buildings. Also, it arguably requires a more skilled installer than is needed when pulling is being used, and it requires more sophisticated installation equipment. Furthermore, this equipment is bulky and its use noisy, and it may be unacceptable to a customer to have this equipment brought on to his premises, should the end from which blowing is to take place be on those premises. Indeed, it may be physically impossible to locate the equipment there. Nevertheless, despite these disadvantages, blowing has been used increasingly, at least in the United Kingdom, for optical fibre unit installation.
JP-A-63-124005 (Sumitomo Electric Industries, Ltd.) describes a modification of the blowing method of EP-A-108590 for installing lightweight fibres. In this, in addition to exerting an installation force on the fibres by means of a flow of air, the fibres are also subjected to a pulling force. It is to be noted that the force exerted by the air is evidently substantial, since the air pressure quoted is 7 kg/cm2, whereas EP-A-108590 stated that a pressure below about 5.6 kg/cm2, usually below half that figure, had been found sufficient to install fibre members of up to 3.5 g/m over lengths of 200 m.
The present invention is based on the discovery that, provided the optical fibre unit is sufficiently lightweight, it can in fact be successfully installed substantially only by pulling, even in routes which are long and which include significant bends, because the low weight per unit length has the result that the required pulling force is below that at which the unit is likely to be damaged, or its optical properties impaired even at the leading end of the fibre. Accordingly, the fibre unit which is proposed herein to be installed by pulling should not include any reinforcement which would have the effect of substantially increasing the weight of the fibre, though, as explained below, it may have lightweight reinforcement.
Thus, according to one aspect of the invention there is provided a method of installing an optical fibre unit in a tube, in which the fibre unit is a lightweight fibre unit, and at least substantially all of the installation force is provided by attaching a pulling member to one end thereof and exerting a pulling force thereon.
The use of pulling not only avoids the disadvantages of blowing referred to above, but generally enables a much higher installation speed to be achieved than is achievable under corresponding conditions using blowing.
The fibre unit preferably has a weight of not more than 10 g/m, and more preferably less than 10 g/m, still more preferably not more than 5 g/m, and for a low fibre count of, say, 8 fibres, is preferably not more than 2 g/m. For a 4 fibre unit might be as little as 0.7 to 0.8 g/m.
The fibre unit may have a protective sheath thereon, for example of an acrylate curable by ultraviolet light, optionally containing particulate matter as described in EP-A-345968 (incorporated herein by reference), to which attention is directed for further details. The fibre unit described in EP-A-345968 is particularly advantageous for present purposes, as the surface thereof, which has protuberances therein as a result of the particulate matter, has been found to provide significantly less resilience to pulling than would otherwise be the case. Materials other than UV-curable acrylate may, however, be used, for example high density polyethylene. However, the thickness of the sheath need be no greater than is required to protect the surface of the optical fibre member and need not be chosen with a view to providing a high tensile strength for the fibre unit.
The fibre unit may have no significant reinforcement, but if it does then the reinforcement must be such as will not add greatly to the weight per unit length of the fibre unit. One form of reinforcement which may be used is a thread made of Kevlar (Registered Trade Mark). A suitable Kevlar thread will preferably not have a weight per unit length of greater than 0.15 g/m, and may have a weight per unit length of as little as 0.05 g/m whilst still providing significant reinforcement.
The tube should be one which exerts a low resistance to the fibre unit being pulled through it, and attention is directed to U.S. Pat. No. 4,892,442 for a description of tubes suitable for this purpose, such tubes having an inner layer which is impregnated with a highly lubricious material. U.S. Pat. No. 4,892,442 (which is incorporated herein by reference) describes, by way of example, the use for the inner layer of Teflon (Registered Trade Mark for polytetra-fluoroethylene) silicone impregnated polyethylene, and graphite impregnated polyethylene. Where an impregnated polyethylene is used this is preferably a high density polyethylene. It should be noted that rather than using a tube lined with a material impregnated with a lubricious agent, one could use a tube formed of such a material throughout its cross section.
In order to insert the lightweight fibre unit into a tube by pulling, it is necessary that the tube should first have a pulling member located therein, and extending along the length of the tube, and that there should be a suitable means for attaching the trailing end of the pulling member to the leading end of the fibre unit.
The first of these requirements can be met by manufacturing a tube and then installing the pulling member therein, for example by blowing. However, it is preferred to manufacture the tube with the pulling member therein. Techniques for manufacturing a large diameter tube (typically about 20-25 mm internal diameter) with a pulling member therein are already known, and involve simultaneously extruding the tube through an annular die and feeding the pulling member through a circular hole which the annular die surrounds. Such techniques can readily be adapted to produce a tube/pulling assembly of a size more appropriate to the installation of a single lightweight optical fibre unit, say one having an internal diameter of from 1 or 1.5 mm to 10 mm. It is to be noted, however, that the fibre unit would normally have a diameter of from 0.5 mm to 1.5 mm, though somewhat larger for high fibre counts (e.g. 19-fibre units), and that the internal diameter of the tube need not exceed the outside diameter of the fibre unit by as large an amount as is needed when blowing is used. In blowing a fibre unit, a relatively large clearance is necessary, at least partly because of the need to ensure that there is an adequate fluid flow path along the whole tube. There are no such constraints when a fibre unit is being pulled. By way of example, an optical fibre unit having an external diameter of 1.1 mm should be capable of satisfactory installation by means of the invention in a tube having an internal diameter of 3.5 mm or less, say as little as about 2.5 mm, over distances of at least 1000 m.
The second of the above requirements, for a suitable means of attaching the pulling member to the fibre unit, can conveniently be met by a braided sheath. Braided sheaths are already known for connecting large diameter cables to pulling members, but such known sheaths are far too large to be suitable for present purposes. However, small diameter braided sheaths are widely available for an entirely unrelated purpose, namely leaders as in fly fishing, and it has been surprisingly found that such leaders can be used, without modification, in the method of the present invention.
In their simplest form, the material for such leaders is supplied to the user as a continuous reel, and the required length is cut off as required. For present purposes, a length of about 40 to 90 mm is sufficient. The ends of the pulling member and optical fibre unit are each inserted in a respective end of the braided sheath, and when tension is exerted in a direction tending to pull the pulling member and optical fibre unit away from one another and out of the sheath, the friction between the pulling member and the sheath and between the fibre unit and the sheath tends to elongate the sheath and thereby cause it to contract and grip the ends. However, friction by itself is not normally sufficient, and additional means, for example a small quantity of adhesive, preferably a rapidly setting adhesive such as a cyanoacrylate, may be used to ensure an adequate grip. Preferably, the length of pulling member inserted in the sheath, and the length of optical fibre unit inserted in the sleeve, is equal to at least 20 times its own diameter.
However, fly fishing leaders are also available as individual sheaths which have one open end and one end formed into a loop. Such leaders are particularly convenient for the purposes of the present invention in conjunction with fibre units which have one or more threads of Kevlar or other material therein. The pulling member can be inserted into the open end, and the Kevlar can be tied to the loop. This ensures that substantially the whole of the tension in the fibre unit is carried in that thread or threads, thus further reducing the risk of damaging the optical fibre member (or members) which remains substantially untensioned. Some optical fibre units are provided with a rip cord, and in such cases the end of the rip cord may be attached to the braided sheath for the purpose of pulling.